The U.S. Pat. No. 3,410,135 discloses a force measuring device where at the bottom of a potlike base member an elastomeric block is arranged having a slightly raised peripheral lip. A force introduction piston having a remarkably smaller diameter than the cylindrical interior of the base member fits with its lower face edge into the recess formed by the raised lip. The gap between the peripheral surface of the piston and the cylindrical inner surface of the base member is relatively large and of such width that upon application of a force no elastomeric material is squeezed out of the base member by the piston. In view of the relatively large gap the piston may assume a certain oblique position with respect to the cylindrical interior on the base member without engaging the surface of the latter. In order to limit such an oblique position of the piston a resilient washer may be provided in the upper region of the annular gap between the piston and the interior surface of the base member. There is no connection between the elastomeric material and the metallic surfaces of the base member and the piston in contact with the elastic material. Without the washer considerable lateral forces will be transmitted to the elastomeric material and cause misreadings of the force measured.
With the washer provided a new member of friction is introduced which in turn deteriorates the measuring results. Since the annular gap is relatively wide elastomeric material will be pressed into it at least upon application of large forces which gives rise to further inaccuracies of the measurement.
The U.S. Pat. No. 4,175,429 discloses a force measuring device comprising a pressure sensitive metal wire embedded in block of vulcanized silicon rubber provided at the bottom of a cylindrical recess in a base member and compressible by means of a movable piston acting as a force introduction member onto the silicon rubber. A considerable space is allowed between the interior cylindrical wall of the base member and the peripheral surface of the piston. In order that a clearance may be maintained as large as possible a sealing ring is provided at the edge of the lower face surface of the piston in contact with the silicon rubber. The sealing ring is in contact with the cylindrical surface of the interior of the base member which results in a considerable friction affecting the accuracy of the force measuring device. Furthermore, any lateral force component of a force applied at an oblique angle onto the force introduction member acts at least partially onto the elastomeric material which agian deteriorates the accuracy of measurement.
The U.S. Pat. No. 3,269,174 discloses a stress sensor comprising a thermistor embedded in block of elastomeric material. When applying forces or pressures at this block there is a temperature increase therein detected by the thermistor and evaluated for force or pressure determination. The response of this known stress sensor is too slow for most practical applications.
The U.S. Pat. No. 4,127,788 discloses a device for detecting deformations in mine roofs wherein an open cylindrical casing having inner and outer threaded surfaces is threaded into the mine roof. A piezo-electric cristal is mounted within the casing to sense the deformation and stresses in the mine roof. The piezoelectric cristal is embedded in a epoxy resin. This device is not adapted for force or pressure measuring purposes since the threaded fixture of the block of resine results in a non-linear inaccurate response.
Among prior publications which are directed to such force or pressure measuring devices are the following publications:
G.J. Dyck, U.S. Pat. No. 4,383,584, May 17, 1983 PA1 A.M. Erichsen, U.S. Pat. No. 1,994,388, Mar. 12, 1935 PA1 R.N. Daugherty, U.S. Pat. No. 4,127,788, Nov. 28, 1978 PA1 H.C. Sonderegger, U.S. Pat. No. 3,582,691, June 1, 1971 PA1 M.K. Gauldie, French Pat. No. 516,321, Apr. 16, 1921 PA1 J. Leonhardt, DE Pat. Pub. No. P2,106,193, Aug. 24, 1972